Shift register

ABSTRACT

In one embodiment of the present invention, a unit circuit of a shift register includes a bootstrap circuit configured with a transistor T 1 , a transistor T 2  and a capacitor, a transistor T 3 , a transistor T 4 , and a reset signal generation circuit. By use of two-phase clock signals and whose high level periods do not overlap with each other, the reset signal generation circuit generates a reset signal which is at a high level in a normal state and changes to a low level when an input signal turns into the high level. During a period that the reset signal is at the high level, transistors T 3  and T 4  perform discharge of a node and pull-down of an output signal. Thus, it is possible to obtain a power-saving shift register that fixes an output signal at a low level in a normal state without allowing a through current to flow therein.

This is a Continuation application of application Ser. No. 12/733,117,filed Feb. 12, 2010, which is a National Phase of PCT Patent ApplicationNo. PCT/JP2008/058926, filed on May 15, 2008, which claims priorityunder 35 U.S.C. §119 to Japanese Patent Application No. 2007-236252,filed on Sep. 12, 2007, the contents of each of which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a shift register, and more particularlyrelates to a shift register which is suitably used as a drive circuit ina display device and an imaging device, or the like.

BACKGROUND ART

An active matrix type display device selects pixel circuits arranged ina two dimensional manner on a row basis and writes voltages in theselected pixel circuits in accordance with display data to display animage. In order to select pixel circuits on a row basis, a shiftregister is used as a scanning signal line drive circuit to sequentiallyshift an output signal based on a clock signal. In a display deviceperforming dot sequential driving, moreover, a data signal line drivecircuit includes a shift register which is similar to that describedabove.

In a liquid crystal display device and the like, occasionally, a drivecircuit for pixel circuits is formed monolithically with the pixelcircuits by a manufacturing process for forming TFTs (Thin FilmTransistors) in the pixel circuits. In this case, preferably, the drivecircuit including a shift register is formed by transistors which areequal in conduction type to the TFTs, in order to reduce manufacturingcost. Moreover, increase of clock signals to be supplied to the shiftregister causes increase of layout area and power consumption of clockinterconnection, and the like. In consideration of such a circumstance,it is necessary to configure a shift register that operates based ontwo-phase clock signals, by use of transistors of an identicalconduction type.

In a shift register configured with an N-channel type transistor, abootstrap circuit shown in FIG. 33 is used for outputting a clock signalwithout changing a voltage level of the clock signal. In the circuitshown in FIG. 33, when an input signal IN turns into a high level, anode N1 is precharged to a voltage potential (VDD−Vth) (VDD: a powersupply voltage, Vth: a threshold voltage of a transistor T1) and atransistor T2 turns into an On state. Thereafter, when the input signalIN turns into a low level, the node N1 turns into a floating state;however, the transistor T2 is maintained at the On state.

In this state, when a clock signal CK changes from the low level to thehigh level, the voltage potential at the node N1 becomes higher than VDDby function of a capacitor C1 provided between a gate terminal and asource terminal of the transistor T2 (a bootstrap effect). Therefore,the clock signal CK having a maximum voltage of VDD passes through thetransistor T2 without voltage drop, and then is output from an outputterminal OUT without change of the voltage level thereof.

In order to configure a shift register for use in a display device andthe like by use of the circuit shown in FIG. 33, it is necessary to adda function of discharging the node N1 and a function of pulling down theoutput signal OUT. With regard to this point, conventionally, there havebeen known the following techniques. As shown in FIG. 34, PatentDocument 1 describes a configuration in that a transistor Q11 dischargesa node N1, based on an output signal from a subsequent circuit, and atransistor Q12 pulls down an output signal OUT, based on a clock signalCK2. As shown in FIG. 35, Patent Document 2 describes a configuration inthat a transistor Q21 discharges a node N1 and transistors Q22 and Q23pull down an output signal OUT, based on an output signal CT from asubsequent circuit.

[Patent Document 1] Japanese Laid-Open Patent Publication No.2001-273785

[Patent Document 2] Japanese Laid-Open Patent Publication No.2002-258819

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In the circuit shown in FIG. 34, however, an output terminal OUTi turnsinto a floating state during a period that the clock signal CK2 is at alow level. Consequently, a shift register erroneously operates becauseof an influence of noise contained in an output signal OUTi in someinstances. In the circuit shown in FIG. 35, a transistor Q24 for pull-upis provided for preventing a node N2 from turning into a floating state.In this circuit, however, a through current flows via the transistor Q24and a transistor Q25 during a period that an input signal IN is at ahigh level and also flows via the transistor Q24 and a transistor Q26during a period that the output signal OUT is at the high level,resulting in increase of power consumption.

Hence, it is an object of the present invention to provide apower-saving shift register capable of fixing an output signal at apredetermined level in a normal state without allowing a through currentto flow therein.

Solutions to the Problems

A first aspect of the present invention provides a shift register havinga configuration in that unit circuits each configured with transistorsof an identical conduction type are cascaded, and operating based ontwo-phase clock signals whose on-level periods do not overlap with eachother, wherein the unit circuit includes: an output control transistorhaving a first conduction terminal supplied with one of the clocksignals, and a second conduction terminal connected to an outputterminal; a precharge circuit for applying an on-voltage to a controlterminal of the output control transistor during a period that an inputsignal is at an on-level; a reset signal generation circuit forgenerating a reset signal which turns into the on-level in a normalstate, by use of the two-phase clock signals, and changing the resetsignal to an off-level when the input signal turns into the on-level;and a discharge circuit for applying an off-voltage to the controlterminal of the output control transistor during a period that the resetsignal is at the on-level.

A second aspect of the present invention provides the shift register inthe first aspect of the present invention, wherein the unit circuitfurther includes an output reset circuit for applying the off-voltage tothe output terminal during the period that the reset signal is at theon-level.

A third aspect of the present invention provides the shift register inthe first aspect of the present invention, wherein the reset signalgeneration circuit includes: a first transistor having a controlterminal supplied with one of the clock signals, and a first conductionterminal applied with the on-voltage; a second transistor having acontrol terminal supplied with the remaining clock signal, and a firstconduction terminal connected to a second conduction terminal of thefirst transistor; and a third transistor having a control terminalsupplied with the input signal, a first conduction terminal connected toa second conduction terminal of the second transistor, and a secondconduction terminal applied with the off-voltage, and the reset signalis output from a connection point between the second and thirdtransistors.

A fourth aspect of the present invention provides the shift register inthe first aspect of the present invention, wherein the unit circuitfurther includes an initialization circuit for fixing the reset signalat the on-level in accordance with an initialization signal.

A fifth aspect of the present invention provides the shift register inthe first aspect of the present invention, wherein the unit circuitfurther includes an additional output control transistor having acontrol terminal and a first conduction terminal connected as in thoseof the output control transistor, and a second conduction terminalconnected to an additional output terminal, and an output signal fromthe output terminal is output externally, and an output signal from theadditional output terminal is supplied to the subsequent unit circuit.

A sixth aspect of the present invention provides the shift register inthe fifth aspect of the present invention, wherein the unit circuitfurther includes an additional output reset circuit for applying theoff-voltage to the additional output terminal during the period that thereset signal is at the on-level.

A seventh aspect of the present invention provides a display deviceincluding: a plurality of pixel circuits arranged in a two dimensionalmanner; and a drive circuit including the shift register according toany one of the first to sixth aspects of the present invention.

Effects of the Invention

According to the first aspect of the present invention, it is possibleto generate the reset signal at the on-level without flow of a throughcurrent by use of the two-phase clock signals whose on-level periods donot overlap with each other, and to apply the off-voltage to the controlterminal of the output control transistor by use of the reset signal.Accordingly, it is possible to fix an output signal at a predeterminedlevel in a normal state without flow of a through current whileachieving low power consumption.

According to the second aspect of the present invention, it is possibleto apply the off-voltage to the output terminal by use of the resetsignal used upon application of the off-voltage to the control terminalof the output control transistor. Accordingly, it is possible to fix anoutput signal at an off-level without flow of a through current whileachieving low power consumption.

According to the third aspect of the present invention, it is possibleto generate the reset signal which is at the on-level in the normalstate and changes to the off-level when the input signal turns into theon-level, by use of fewer transistors.

According to the fourth aspect of the present invention, it is possibleto turn the reset signal into the on-level and to turn all the outputsignals into the off-level in such a manner that the initializationsignal is supplied externally upon initialization.

According to the fifth aspect of the present invention, it is possibleto prevent erroneous operations of the shift register in such a mannerthat the external output signal and the input signal to the subsequentunit circuit are output from the unit circuit in a separate manner.

According to the sixth aspect of the present invention, it is possibleto apply the off-voltage to the additional output terminal by use of thereset signal used upon application of the off-voltage to the controlterminal of the output control transistor. Accordingly, it is possibleto fix an additional output signal at an off-level without flow of athrough current while achieving low power consumption.

According to the seventh aspect of the present invention, it is possibleto obtain a power-saving display device by use of a drive circuitincluding a power-saving shift register.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a shift registeraccording to a first embodiment of the present invention.

a. FIG. 2 is a circuit diagram of a unit circuit included in the shiftregister shown in FIG. 1.

b. FIG. 3 is a timing chart of the shift register shown in FIG. 1.

c. FIG. 4 is a block diagram showing a configuration of a liquid crystaldisplay device including the shift register shown in FIG. 1.

d. FIG. 5 is a block diagram showing a configuration of a differentliquid crystal display device including the shift register shown in FIG.1.

e. FIG. 6 is a timing chart upon initialization of the shift registershown in FIG. 1.

f. FIG. 7 is a block diagram showing a configuration of a shift registeraccording to a second embodiment of the present invention.

g. FIG. 8 is a circuit diagram of a unit circuit included in the shiftregister shown in FIG. 7.

h. FIG. 9 is a block diagram showing a configuration of a shift registeraccording to a third embodiment of the present invention.

i. FIG. 10 is a circuit diagram of a unit circuit included in the shiftregister shown in FIG. 9.

j. FIG. 11 is a timing chart of the shift register shown in FIG. 9.

k. FIG. 12 is a circuit diagram of a different initialization circuitincluded in the shift register shown in FIG. 9.

l. FIG. 13 is a circuit diagram of a unit circuit included in a shiftregister according to a fourth embodiment of the present invention.

m. FIG. 14 is a block diagram showing a configuration of a shiftregister according to a fifth embodiment of the present invention.

n. FIG. 15 is a circuit diagram of a unit circuit included in the shiftregister shown in FIG. 14.

FIG. 16 is a block diagram showing a configuration of a shift registeraccording to a sixth embodiment of the present invention.

p. FIG. 17 is a circuit diagram of a unit circuit included in the shiftregister shown in FIG. 16.

q. FIG. 18 is a timing chart of the shift register shown in FIG. 16.

r. FIG. 19 is a block diagram showing a configuration of a shiftregister according to a seventh embodiment of the present invention.

s. FIG. 20 is a circuit diagram of a unit circuit included in the shiftregister shown in FIG. 19.

t. FIG. 21 is a timing chart of the shift register shown in FIG. 19.

u. FIG. 22 is a block diagram showing a configuration of a shiftregister according to an eighth embodiment of the present invention.

v. FIG. 23 is a circuit diagram of a unit circuit included in the shiftregister shown in FIG. 22.

w. FIG. 24 is a timing chart of the shift register shown in FIG. 22.

x. FIG. 25 is a circuit diagram of a precharge circuit included in ashift register according to a first modification example of the presentinvention.

y. FIG. 26 is a circuit diagram of a different precharge circuitincluded in the shift register according to the first modificationexample of the present invention.

z. FIG. 27 is a circuit diagram of a unit circuit included in a shiftregister according to a second modification example of the presentinvention.

aa. FIG. 28 is a circuit diagram of a unit circuit included in a shiftregister according to a third modification example of the presentinvention.

bb. FIG. 29 is a timing chart of the shift register according to thethird modification example of the present invention.

cc. FIG. 30 is a block diagram showing a configuration of a shiftregister according to a fourth modification example of the presentinvention.

dd. FIG. 31 is a circuit diagram of a unit circuit included in a shiftregister according to a fifth modification example of the presentinvention.

ee. FIG. 32 is a timing chart of the shift register according to thefifth modification example of the present invention.

ff. FIG. 33 is a circuit diagram of a bootstrap circuit included in ashift register.

gg. FIG. 34 is a circuit diagram showing a configuration of aconventional shift register.

hh. FIG. 35 is a circuit diagram showing a configuration of a differentconventional shift register.

DESCRIPTION OF REFERENCE SYMBOLS

-   10, 20, 30, 50, 60, 70, 80, 90 Shift register-   ii. 11, 17, 18, 19, 21, 31, 41, 51, 61, 71, 81 Unit circuit-   jj. 12 Reset signal generation circuit-   kk. 13, 15, 16 Precharge circuit-   ll. 22 Scanning direction switch circuit-   mm. 32, 33 Initialization circuit-   nn. 82 Buffer-   oo. 110, 120 Liquid crystal display device-   pp. 111, 121 Pixel array-   qq. 112, 122 Display control circuit-   rr. 113, 123 Scanning signal line drive circuit-   ss. 114, 124 Data signal line drive circuit-   tt. 125 Sampling switch

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

uu. FIG. 1 is a block diagram showing a configuration of a shiftregister according to a first embodiment of the present invention. Theshift register 10 shown in FIG. 1 has a configuration in that n (n: aninteger of two or more) unit circuits 11 are cascaded. The unit circuit11 has a clock terminal CK, a clock terminal CKB, an input terminal INand an output terminal OUT. In the following, signals to be input/outputvia the respective terminals are described using designations equal tothe relevant terminals. (For example, a signal to be input via the clockterminal CK is referred to as a clock signal CK.)

A start pulse ST and two-phase clock signals CK1 and CK2 are suppliedexternally to the shift register 10. The start pulse ST is supplied tothe input terminal IN of the first unit circuit 11. The clock signal CK1is supplied to the clock terminal CK of the odd-numbered unit circuit 11and the clock terminal CKB of the even-numbered unit circuit 11. Theclock signal CK2 is supplied to the clock terminal CKB of theodd-numbered unit circuit 11 and the clock terminal CK of theeven-numbered unit circuit 11. Output signals OUT from the unit circuits11 are output externally as output signals SROUT1 to SROUTn. Moreover,the output signal OUT from the unit circuit 11 is supplied to the inputterminal IN of the subsequent unit circuit 11.

FIG. 2 is a circuit diagram of the unit circuit 11 included in the shiftregister 10. As shown in FIG. 2, the unit circuit 11 is configured withtransistors of an identical conduction type, and includes sevenN-channel type transistors T1 to T7 and three capacitors C1 to C3. Inthe following, a voltage (a level of a signal) which is applied to agate terminal to turn a transistor into an On state is referred to as anon-voltage (an on-level), and a voltage (a level of a signal) which isapplied to the gate terminal to turn the transistor into an Off state isreferred to as an off-voltage (an off-level). In an N-channel typetransistor, a High voltage corresponds to the on-voltage (a high levelcorresponds to the on-level), and a Low voltage corresponds to theoff-voltage (a low level corresponds to the off-level). In a P-channeltype transistor, these relations are reversed.

The transistor T1 has a drain terminal applied with a power supplyvoltage VDD, and a gate terminal connected to the input terminal IN. Asource terminal of the transistor T1 is connected to a gate terminal ofthe transistor T2 and a drain terminal of the transistor T4. In thefollowing, this connection point is referred to as a node N1. Thetransistor T2 has a drain terminal connected to the clock terminal CK,and a source terminal connected to the output terminal OUT and a drainterminal of the transistor T3. Source terminals of the transistors T3and T4 are connected to a ground.

The transistor T5 has a drain terminal applied with the power supplyvoltage VDD, and a source terminal connected to a drain terminal of thetransistor T6. A source terminal of the transistor T6 is connected to adrain terminal of the transistor T7. A source terminal of the transistorT7 is connected to the ground. Gate terminals of the transistors T5 toT7 are connected to the clock terminal CK, the clock terminal CKB andthe input terminal IN, respectively. A connection point between thetransistors T6 and T7 is connected to the gate terminals of thetransistors T3 and T4. In the following, this connection point isreferred to as a node N2 and a connection point between the transistorsT5 and T6 is referred to as a node N3.

Each of the capacitors C1 to C3 is a capacitive element. The capacitorC1 is provided between the gate terminal and the source terminal of thetransistor T2, the capacitor C2 is provided between the node N3 and theground, and the capacitor C3 is provided between the node N2 and theground. The capacitor C1 functions as a bootstrap capacitor, and each ofthe capacitors C2 and C3 functions as a charge pump capacitor. In thefollowing, it is assumed that the capacitor C2 is equal in capacitancevalue to the capacitor C3.

In the unit circuit 11, the transistors T5 to T7 and the capacitors C2and C3 form a reset signal generation circuit 12. Moreover, thetransistors T1 to T4 function as a precharge circuit 13, an outputcontrol transistor, an output reset circuit and a discharge circuit,respectively. The transistor T2 switches whether to output the clocksignal CK from the output terminal OUT, in accordance with a voltagepotential at the gate terminal. During a period that the input signal INis at the high level, the transistor T1 applies the High voltage to thenode N1 (the gate terminal of the transistor T2). The reset signalgeneration circuit 12 generates a reset signal which is at the highlevel in a normal state and changes to the low level when the inputsignal IN turns into the high level. During a period that the resetsignal is at the high level, the transistor T4 applies the Low voltageto the node N1. During the period that the reset signal is at the highlevel, the transistor T3 applies the Low voltage to the output terminalOUT.

FIG. 3 is a timing chart of the shift register 10. In FIG. 3, periods t0to tn+1 correspond to one line period (one horizontal period), and eachperiod is divided into a first half and a second half. The start pulseST turns into the high level in the first half of the period t0, theclock signal CK1 turns into the high level in the first half of theperiod to (o is an odd number; hereinafter, referred to as an oddperiod), and the clock signal CK2 turns into the high level in the firsthalf of the period to (e is an even number; hereinafter, referred to asan even period). These three signals turn into the low level during theremaining periods. As described above, the clock signals CK1 and CK2have a characteristic in that the respective high level periods do notoverlap with each other. In the following, it is assumed that all theinput/output signals and signals in the shift register 10, including theclock signals CK1 and CK2, have a voltage potential which is VDD at thehigh level and VSS (zero) at the low level unless otherwise specified.

Upon reception of the input signal shown in FIG. 3, the first unitcircuit 11 (hereinafter, referred to as the unit circuit SR1) operatesas follows. In the unit circuit SR1, the input signal IN turns into thehigh level in the first half of the period t0, the clock signal CK turnsinto the high level in the first half of the odd period, and the clocksignal CKB turns into the high level in the first half of the evenperiod.

Prior to the period t0, the input signal IN is at the low level;therefore, the transistors T1 and T7 are in the Off state. Herein, avoltage potential at the node N2 and a voltage potential at the node N3are VDD (the reason therefor will be described later); therefore, thetransistors T3 and T4 are in the On state. Accordingly, a voltagepotential at the node N1 and a voltage potential at the output terminalOUT are VSS, and the transistor T2 is in the Off state. At this point intime, no electrical charge is stored in the capacitor C1, and electricalcharge is stored in each of the capacitors C2 and C3 in accordance withthe power supply voltage VDD.

In the first half of the period t0, the input signal IN and the clocksignal CKB turn into the high level; therefore, the transistors T1, T6and T7 turn into the On state. Thus, the electrical charge stored ineach of the capacitors C2 and C3 is discharged, the voltage potentialsat the nodes N2 and N3 turn into VSS, and transistors T3 and T4 turninto the Off state. When the transistor T1 turns into the On state, thevoltage potential at the node N1 turns into (VDD−Vth) (Vth: a thresholdvoltage of the transistor T1), so that the transistor T2 turns into theOn state. Herein, the clock signal CK is at the low level; therefore,the output signal OUT is maintained at the low level. Thus, electricalcharge is stored in the capacitor C1 in accordance with a gate-sourcepotential difference (VDD−Vth) of the transistor T2.

In the second half of the period t0, the input signal IN and the clocksignal CKB turn into the low level; therefore, the transistors T1, T6and T7 turn into the Off state. When the transistor T1 turns into theOff state, the node N1 turns into a floating state; however, the voltagepotential at the node N1 is held at (VDD−Vth) by the capacitor C1.

In the first half of the period t1, the clock signal CK turns into thehigh level. Herein, the transistor T2 is in the On state; therefore, theoutput signal OUT also turns into the high level. The node N1 is in thefloating state, and the node N1 and the source terminal of thetransistor T2 are connected via the capacitor C1 that holds thepotential difference (VDD−Vth); therefore, the voltage potential at thesource terminal of the transistor T2 changes from VSS to VDD, so thatthe voltage potential at the node N1 changes by the same amount andbecomes higher than the power supply voltage VDD (a bootstrap effect).Thus, the clock signal CK having a maximum voltage of VDD passes throughthe transistor T2 without voltage drop, and is output from the outputterminal OUT without change of a voltage level thereof. Moreover, whenthe clock signal CK turns into the high level, the transistor T5 turnsinto the On state. Herein, the transistor T6 is in the Off state;therefore, the voltage potential at the node N3 turns into VDD, so thatelectrical charge is stored in the capacitor C2 in accordance with thepower supply voltage VDD.

In the second half of the period t1, the clock signal CK turns into thelow level. Herein, the transistor T2 is in the On state; therefore, theoutput signal OUT also turns into the low level, and the voltagepotential at the node N1 returns to (VDD−Vth). Moreover, the transistorT5 turns into the Off state. At the end of the period t1, the voltagepotential at the node N2 is VSS and the voltage potential at the node N3is VDD.

In the first half of the period t2, the clock signal CKB turns into thehigh level; therefore, the transistor T6 turns into the On state.Herein, the electrical charge stored in the capacitor C2 partly moves tothe capacitor C3, so that the voltage potential at the node N2 rises. Inthe case where the capacitor C2 is equal in capacitance value to thecapacitor C3, the node N2 becomes equal in voltage potential to the nodeN3, and the voltage potential at the node N2 rises to VDD/2. Thecapacitance value of the capacitors C2 and C3 is determined such thatthe voltage potential at the node N2 at this point in time becomeshigher than threshold voltages at the transistors T3 and T4. In thefirst half of the period t2, thus, the transistors T3 and T4 turn intothe On state, and the voltage potential at the node N1 and the voltagepotential at the output terminal OUT turn into VSS.

Thereafter, the reset signal generation circuit 12 in the unit circuitSR1 operates as follows. In the first half of the odd period, the clocksignal CK turns into the high level and the clock signal CKB turns intothe low level; therefore, the transistor T5 turns into the On state andthe transistor T6 turns into the Off state. Herein, the voltagepotential at the node N3 turns into VDD and electrical charge is storedin the capacitor C2 in accordance with the power supply voltage VDD. Inthe first half of the even period, on the other hand, the clock signalCK turns into the low level and the clock signal CKB turns into the highlevel; therefore, the transistor T5 turns into the Off state and thetransistor T6 turns into the On state. Herein, the electrical chargestored in the capacitor C2 partly moves to the capacitor C3, so that thevoltage potential at the node N2 rises. In the case where the capacitorC2 is equal in capacitance value to the capacitor C3, the voltagepotential at the node N2 gradually rises to eventually reach VDD.

As shown in FIG. 3, thus, the voltage potential (designated by SR1_N1;hereinafter, this symbol designates the same meaning) at the node N1 inthe unit circuit SR1 turns into (VDD−Vth) in the period t0 and thesecond half of the period t1, becomes higher than VDD in the first halfof the period t1, and turns into VSS in the remaining periods. Thevoltage potential at the node N2 in the unit circuit SR1 turns into VSSin the periods t0 and t1, and gradually rises in the period t2 and theperiods subsequent to the period t2 to eventually turn into VDD. Theoutput signal OUT from the unit circuit SR1 (the output signal SROUT1from the shift register 10) turns into the high level in the first halfof the period t1 and turns into the low level in the remaining periods.

Likewise, the output signal OUT from the i-th (i: an integer in a rangeof one or more to n or less) unit circuit 11 (the output signal SROUTifrom the shift register 10) turns into the high level in the first halfof the period t1 and turns into the low level in the remaining periods.As described above, the shift register 10 sequentially turns the outputsignals SROUT1 to SROUTn into the high level one by one, based on thetwo-phase clock signals CK1 and CK2.

The shift register 10 is used as a drive circuit in a display device andan imaging device, or the like, for example. FIG. 4 is a block diagramshowing a configuration of a liquid crystal display device including theshift register 10. The liquid crystal display device 110 shown in FIG. 4is an active matrix type display device including a pixel array 111, adisplay control circuit 112, a scanning signal line drive circuit 113and a data signal line drive circuit 114. In the liquid crystal displaydevice 110, the shift register 10 is used as the scanning signal linedrive circuit 113.

The pixel array 111 shown in FIG. 4 includes n scanning signal lines G1to Gn, m data signal lines S1 to Sm, and (m×n) pixel circuits Pij (m: aninteger of two or more, j: an integer in a range of one or more to m orless). The scanning signal lines G1 to Gn are arranged in parallel withone another, and the data signal lines S1 to Sm are arranged in parallelwith one another so as to be orthogonal to the scanning signal lines G1to Gn. The pixel circuit Pij is arranged near an intersection of thescanning signal line Gi and the data signal line Sj. As described above,the (m×n) pixel circuits Pij are arranged in a two dimensional manner inwhich m pixel circuits are arranged in a row direction while n pixelcircuits are arranged in a column direction. The scanning signal line Giis connected to all the pixel circuits Pij on the i-th row, and the datasignal line Sj is connected to all the pixel circuits Pij on the j-thcolumn.

The liquid crystal display device 110 is externally supplied withcontrol signals such as a horizontal synchronous signal HSYNC and avertical synchronous signal VSYNC, and display data DT. Based on thesesignals, the display control circuit 112 outputs a clock signal CK1, aclock signal CK2 and a start pulse ST to the scanning signal line drivecircuit 113, and outputs a control signal SC and the display data DT tothe data signal line drive circuit 114.

The scanning signal line drive circuit 113 is configured with the shiftregister 10 having n stages. Based on the clock signals CK1 and CK2, theshift register 10 sequentially turns the output signals SROUT1 to SROUTninto the high level (indicating a selection state) one by one. Theoutput signals SROUT1 to SROUTn are supplied to the scanning signallines G1 to Gn, respectively. Thus, the scanning signal lines G1 to Gnare sequentially selected one by one, so that the pixel circuits Pij onone row are collectively selected.

Based on the control signal SC and the display data DT, the data signalline drive circuit 114 applies voltages according to the display data DTto the data signal lines S1 to Sm. Thus, the voltages according to thedisplay data DT are written in the selected pixel circuits Pij on onerow. By the operations described above, the liquid crystal displaydevice 110 displays an image.

FIG. 5 is a block diagram showing a configuration of a different liquidcrystal display device including the shift register 10. The liquidcrystal display device 120 shown in FIG. 5 is an active matrix typedisplay device including a pixel array 121, a display control circuit122, a scanning signal line drive circuit 123 and a data signal linedrive circuit 124. In the liquid crystal display device 120, the shiftregister 10 is used while being incorporated in the data signal linedrive circuit 124 that performs dot sequential driving.

The pixel array 121 shown in FIG. 5 is similar in configuration to thepixel array 111 shown in FIG. 4. However, the pixel array 121 includes mscanning signal lines and n data signal lines, and (m×n) pixel circuitsPij are arranged in a two-dimensional manner in which n pixel circuitsare arranged in a row direction while m pixel circuits are arranged in acolumn direction.

Based on the control signals and the display data DT each givenexternally, the display control circuit 122 outputs a control signal GCto the scanning signal line drive circuit 123, and outputs a clocksignal CK1, a clock signal CK2, a start pulse ST and analog display dataADT to the data signal line drive circuit 124. Based on the controlsignal GC, the scanning signal line drive circuit 123 sequentiallyselects the scanning signal lines G1 to Gm one by one.

The data signal line drive circuit 124 includes the shift register 10having the n stages, and n sampling switches 125. The n samplingswitches 125 have first ends connected to the data signal lines S1 toSn, respectively, and second ends supplied with the analog display dataADT. The n sampling switches 125 have gate terminals supplied with theoutput signals SROUT1 to SROUTn from the shift register 10,respectively.

The output signals SROUT1 to SROUTn sequentially turn into the highlevel one by one; therefore, the n sampling switches 125 sequentiallyturn into the On state one by one, and the analog display data ADT issupplied to the data signal line connected to the sampling switch 125 inthe On state. Thus, voltages according to the display data DT aresequentially written in the pixel circuits Pij on one row selected bythe scanning signal line drive circuit 123 one by one. By the operationsdescribed above, the liquid crystal display device 120 displays animage. Herein, one output signal SROUTi may be supplied to the gateterminals of the plurality of sampling switches.

As described above, the shift register 10 is used as the scanning signalline drive circuit of the display device or is used while beingincorporated in the data signal line drive circuit of the displaydevice. In addition to this, the shift register 10 is used as a drivecircuit in an imaging device, or the like. By use of the shift register10 in the display device, the imaging device and the like, it ispossible to correctly drive scanning signal lines or data signal lines.

Hereinafter, description will be given of effects of the shift register10 according to this embodiment. As described above, the reset signalgeneration circuit 12 in the unit circuit 11 generates the reset signalat the high level, based on the two-phase clock signals CK1 and CK2whose high level periods do not overlap with each other, and changes thereset signal to the low level when the input signal IN turns into thehigh level. By use of the reset signal, the unit circuit 11 applies theLow voltage to the gate terminal of the transistor T2 and the outputterminal OUT. (That is, the unit circuit 11 performs the discharge ofthe node N1 and the pull-down of the output signal OUT.)

In the shift register 10, as described above, the high level periods ofthe clock signals CK1 and CK2 do not overlap with each other; therefore,there are no possibilities that the transistors T5 and T6 simultaneouslyturn into the On state and the through current flows in the transistorsT5 and T6. For this reason, it is possible to generate the reset signalat the high level without flow of the through current and to perform thedischarge of the node N1 and the pull-down of the output signal OUT byuse of this reset signal. Accordingly, the shift register 10 can fix theoutput signal OUT at the low level in the normal state without allowingthe through current to flow therein while achieving low powerconsumption.

In the shift register 10, further, it is possible to generate the resetsignal in the unit circuit 11 without use of the output signal from thesubsequent unit circuit and to apply the Low voltage to the gateterminal of the transistor T2 and the output terminal OUT by use of thisreset signal. Therefore, it is unnecessary to use the output signal fromthe subsequent circuit in order to perform the discharge of the node N1and the pull-down of the output signal OUT. Accordingly, the shiftregister 10 allows reduction of interconnections between circuits, andalso allows reduction of layout area and power consumption thereof.Moreover, the shift register 10 uses no output signal from thesubsequent circuit, and therefore operates in a stable manner.

The reset signal generation circuit 12 is configured with thetransistors T5 to T7 and the capacitors C2 and C3, so that the resetsignal, which is at the high level in the normal state and changes tothe low level when the input signal IN turns into the high level, can begenerated using fewer transistors.

In the period t shown in FIG. 6, when the start pulse ST at the lowlevel and the clock signals CK1 and CK2 at the high level are suppliedto the shift register 10, the voltage potential at the node N2 turnsinto VDD whereas the voltage potential at the node N1 and the voltagepotential at the output terminal OUT turn into VSS in the unit circuit11. Thus, the shift register 10 can perform the discharge of the node N1and the pull-down of the output signal OUT, when both the clock signalsCK1 and CK2 turn into the high level.

Shift registers according to second to eighth embodiments are similar inconfiguration to the shift register 10, and are used as in the shiftregister 10. Hereinafter, description will be given of points ofdifference between the following embodiments and the first embodiment,and description of commonalities between the following embodiments andthe first embodiment will not be given.

Second Embodiment

vv. FIG. 7 is a block diagram showing a configuration of a shiftregister according to a second embodiment of the present invention. Theshift register 20 shown in FIG. 7 includes n unit circuits 21 eachhaving a scanning direction switch terminal UD, a scanning directionswitch terminal UDB, an input terminal INu and an input terminal INd.The shift register 20 corresponds to the shift register 10 according tothe first embodiment to which a function of switching a scanningdirection (a direction of shift of an output signal) is added.

In the shift register 20, a direction along which a number of the unitcircuit 21 becomes large (a downward direction in FIG. 7) is referred toas a forward direction, and a reverse direction thereof (an upwarddirection in FIG. 7) is referred to as a backward direction. The unitcircuit 21 having the number which is smaller by one is referred to as aforward unit circuit, and the unit circuit 21 having the number which islarger by one is referred to as a rearward unit circuit. When thescanning direction is the forward direction, the forward unit circuitcorresponds to a preceding unit circuit whereas the rearward unitcircuit corresponds to a subsequent unit circuit. On the other hand,when the scanning direction is the backward direction, the forward unitcircuit corresponds to a subsequent unit circuit whereas the rearwardunit circuit corresponds to a preceding unit circuit.

A start pulse ST is supplied to the input terminal INu of the first unitcircuit 21 and the input terminal INd of the n-th unit circuit 21. Anoutput signal OUT from the unit circuit 21 is supplied to the inputterminal INu of the rearward unit circuit 21 and the input terminal INdof the forward unit circuit 21. Scanning direction switch signals UD andUDB (negation of UD) each given externally are supplied to the scanningdirection switch terminals UD and UDB of the unit circuit 21,respectively.

FIG. 8 is a circuit diagram of the unit circuit 21 included in the shiftregister 20. The unit circuit 21 shown in FIG. 8 corresponds to the unitcircuit 11 according to the first embodiment to which a scanningdirection switch circuit 22 including two N-channel type transistors T11and T12 is added. The transistors T11 and T12 have drain terminalsconnected to the input terminals INu and INd, respectively, and gateterminals connected to the scanning direction switch terminals UD andUDB, respectively. The transistors T11 and T12 have source terminalseach connected to an output terminal of the scanning direction switchcircuit 22, and this output terminal is connected to a gate terminal ofa transistor T1 and a gate terminal of a transistor T7.

When the scanning direction switch signal UD is at a high level and thescanning direction switch signal UDB is at a low level, the transistorT11 turns into an On state, the transistor T12 turns into an Off state,and the gate terminals of the transistors T1 and T7 are connected to theinput terminal INu. Herein, the unit circuit 21 receives an outputsignal from the forward unit circuit 21, and the shift register 20sequentially shifts the output signal in the forward direction.

When the scanning direction switch signal UD is at the low level and thescanning direction switch signal UDB is at the high level, thetransistor T11 turns into the Off state, the transistor T12 turns intothe On state, and the gate terminals of the transistors T1 and T7 areconnected to the input terminal INd. Herein, the unit circuit 21receives an output signal from the rearward unit circuit 21, and theshift register sequentially shifts the output signal in the backwarddirection. In a case where n is an even number, there arises a necessitythat high level periods of clock signals CK1 and CK2 are reversed inorder to shift the output signal in the backward direction.

The shift register 20 according to this embodiment selects, as an inputsignal, either the output signal from the forward unit circuit (theinput signal INu) or the output signal from the rearward unit circuit(the input signal INd), by use of the scanning direction switch circuit22, to switch the scanning direction without provision of a dummy stage.

In a conventional shift register that uses an output signal from asubsequent circuit in order to perform discharge of a node N1 andpull-down of an output signal OUT, there is a necessity that one unitcircuit includes two circuits similar to the scanning direction switchcircuit 22. In the shift register 20, on the other hand, the unitcircuit 21 requires only one scanning direction switch circuit 22. Thus,the shift register 20 allows reduction of an amount of circuits in ashift register having a scanning direction switching function.

Third Embodiment

ww. FIG. 9 is a block diagram showing a configuration of a shiftregister according to a third embodiment of the present invention. Theshift register 30 shown in FIG. 9 includes n unit circuits 31 eachhaving an initialization terminal INIT. The shift register 30corresponds to the shift register 10 according to the first embodimentto which an initializing function is added. In the unit circuit 31, theinitialization terminal INIT is supplied with an initialization signalINIT given externally.

FIG. 10 is a circuit diagram of the unit circuit 31 included in theshift register 30. The unit circuit 31 shown in FIG. 10 corresponds tothe unit circuit 11 according to the first embodiment to which aninitialization circuit 32 including an N-channel type transistor T13 isadded. The transistor T13 has a drain terminal applied with a powersupply voltage VDD, a source terminal connected to a node N2, and a gateterminal connected to the initialization terminal INIT.

When the initialization signal INIT is at a low level, the transistorT13 turns into an Off state, and the unit circuit 31 operates as in theunit circuit 11 according to the first embodiment. When theinitialization signal INIT is at a high level, the transistor T13 turnsinto an On state, so that a voltage potential at the node N2 turns intoVDD. Therefore, transistors T3 and T4 turn into the On state, and avoltage potential at a node N1 and a voltage potential at an outputterminal OUT turn into VSS.

FIG. 11 is a timing chart of the shift register 30. As shown in FIG. 11,before initialization, the voltage potential at each of the node N1, thenode N2 and the output terminal OUT is not fixed in the unit circuit 31(shown with a cross mark). During a period t that the initializationsignal INIT turns into the high level, the voltage potential at the nodeN2 turns into VDD, and the voltage potential at the node N1 and thevoltage potential at the output terminal OUT turn into VSS.

The shift register 30 according to this embodiment can turn a resetsignal into the high level and turn all output signals SROUT1 to SROUTninto the low level when the initialization signal INIT is suppliedexternally upon initialization.

The unit circuit 31 may include an initialization circuit 33 shown inFIG. 12 in place of the initialization circuit 32. In the initializationcircuit 33, a transistor T13 has a drain terminal and a gate terminaleach connected to an initialization terminal INIT. Also in the case ofusing the initialization circuit 33, during the period that theinitialization signal INIT is at the high level, the transistor T13turns into the On state, the voltage potential at the node N2 turns intoVDD, and the voltage potential at the node N1 and the voltage potentialat the output terminal OUT turn into VSS. By use of the initializationcircuit 33, moreover, it is possible to reduce a stress due to agate-drain voltage.

Fourth Embodiment

xx. A shift register according to a fourth embodiment of the presentinvention is identical in configuration to that according to the firstembodiment (FIG. 1). The shift register according to this embodimentcorresponds to the shift register 10 according to the first embodimentto which a function of fixing a reset signal at a low level during aperiod that an output signal is at a high level is added.

FIG. 13 is a circuit diagram of a unit circuit 41 included in the shiftregister according to this embodiment. The unit circuit 41 shown in FIG.13 corresponds to the unit circuit 11 according to the first embodimentto which a transistor T14 functioning as a reset signal fixing circuitis added. The transistor T14 has a drain terminal connected to a nodeN2, a gate terminal connected to an output terminal OUT, and a sourceterminal connected to a ground. During a period that the output terminalOUT is applied with a High voltage, the transistor T14 turns into an Onstate, and the node N2 is applied with a Low voltage (i.e., the resetsignal is fixed at the low level).

In the unit circuit 11 according to the first embodiment, when the inputsignal IN changes to the low level, the node N2 turns into a floatingstate until the clock signal CKB changes to the high level. Herein, thevoltage potential at the node N2 must be VSS in a usual case, butoccasionally varies because of an influence of noise and the like. Forthis reason, there is a possibility that the voltage potential at thenode N2 rises and the transistor T3 turns into the On state, so that theoutput signal OUT can not be output correctly.

In order to solve this problem, the unit circuit 41 includes thetransistor T14 (the reset signal fixing circuit) that turns the resetsignal into the low level during the period that the output signal OUTis at the high level. Thus, the shift register according to thisembodiment can prevent erroneous operations by fixing the reset signalat the low level during the period that the output signal OUT is at thehigh level.

Fifth Embodiment

yy. FIG. 14 is a block diagram showing a configuration of a shiftregister according to a fifth embodiment of the present invention. Theshift register 50 shown in FIG. 14 includes n unit circuits 51 eachhaving an output terminal OUT1 and an output terminal OUT2. The shiftregister 50 corresponds to the shift register 10 according to the firstembodiment to which a function of outputting a plurality of identicaloutput signals (an output separating function) is added.

In the shift register 50, identical output signals are output from theoutput terminals OUT1 and OUT2 of the unit circuit 51. The outputsignals OUT1 from the unit circuits 51 are externally output as outputsignals SROUT1 to SROUTn. The output signals OUT2 from the unit circuits51 except the n-th unit circuit 51 are supplied to input terminals IN ofthe subsequent unit circuits 51. The output signal OUT2 from the n-thunit circuit 51 is externally output as a test output signal TESTOUT.

FIG. 15 is a circuit diagram of the unit circuit 51 included in theshift register 50. The unit circuit 51 shown in FIG. 15 corresponds tothe unit circuit 11 according to the first embodiment to which atransistor T15, a transistor T16 and a capacitor C4 are added. Thetransistor T15 functions as an additional output control transistor. Thetransistor T16 functions as an additional output reset circuit thatapplies a Low voltage to the output terminal OUT2 during a period that areset signal is at a high level.

In the unit circuit 51, a connection point between transistors T2 and T3is connected to the output terminal OUT1. The transistor T15 has a drainterminal and a gate terminal connected as in those of the transistor T2.More specifically, the transistor T15 has the drain terminal connectedto a clock terminal CK, and the gate terminal connected to a node N1. Asource terminal of the transistor T15 is connected to the outputterminal OUT2 and a drain terminal of the transistor T16. The transistorT16 has a source terminal connected to a ground, and a gate terminalconnected to a node N2. The capacitor C4 is provided between the gateterminal and the source terminal of the transistor T15.

In the shift register 50, the external output signal (the output signalOUT1) and the input signal (the output signal OUT2) to the subsequentunit circuit are output from the unit circuit 51 in a separate manner.Therefore, even when a level of the output signal OUT1 varies because ofan influence of external noise and the like, this influence is notexerted on the input signal to the subsequent unit circuit. Thus, theshift register 50 according to this embodiment can prevent erroneousoperations by outputting the external output signal and the input signalto the subsequent unit circuit in a separate manner.

The unit circuit 51 may include both of or one of a capacitor C1 and thecapacitor C4. A capacitance value of the capacitor C4 is set to becomesmall, leading to reduction of an influence to be exerted on the outputsignal OUT2 because of level variations of clock signals CK1 and CK2.Moreover, a capacitance value of the capacitor C1 is set to becomesmall, leading to reduction of an influence to be exerted on the outputsignal OUT2 because of a level variation of the output signal OUT1. Inview of these points, the capacitance values of the capacitors C1 and C4are determined appropriately, so that it is possible to operate theshift register 50 in an optimal state while reducing an influence ofexternal noise.

Moreover, since the n-th output signal OUT2 is output as the test outputsignal TESTOUT, it is possible to output a test output signal withoutchanging a load of the n-th output signal OUT1 and without providing adummy stage.

Sixth Embodiment

zz. FIG. 16 is a block diagram showing a configuration of a shiftregister according to a sixth embodiment of the present invention. Theshift register 60 shown in FIG. 16 includes n unit circuits 61 eachhaving an all-on control terminal AON and an all-on control terminalAONB. The shift register 60 corresponds to the shift register 10according to the first embodiment to which a function of turning alloutput signals into a high level (an all-on function) is added. In theunit circuit 61, the all-on control terminals AON and AONB are suppliedwith all-on control signals AON and AONB (negation of AON) each givenexternally.

FIG. 17 is a circuit diagram of the unit circuit 61 included in theshift register 60. The unit circuit 61 shown in FIG. 17 corresponds tothe unit circuit 11 according to the first embodiment to whichtransistors T17 and T18 each functioning as an all-on control circuitare added. The transistor T17 has a drain terminal applied with a powersupply voltage VDD, a source terminal connected to a drain terminal of atransistor T5, and a gate terminal connected to the all-on controlterminal AONB. The transistor T18 has a drain terminal applied with thepower supply voltage VDD, a source terminal connected to an outputterminal OUT, and a gate terminal connected to the all-on controlterminal AON.

When the all-on control signal AON is at a low level and the all-oncontrol signal AONB is at the high level, the transistor T17 turns intoan On state, the transistor T18 turns into an Off state, and the unitcircuit 61 operates as in the unit circuit 11 according to the firstembodiment. When the all-on control signal AON is at the high level andthe all-on control signal AONB is at the low level, the transistor T17turns into the Off state, and the transistor T18 turns into the Onstate. Herein, a transistor T7 turns into the On state, a voltagepotential at a node N2 turns into VSS, and a voltage potential at anoutput terminal OUT turns into VDD.

FIG. 18 is a timing chart of the shift register 60. In FIG. 18, during aperiod t that the all-on control signal AON and a start pulse ST turninto the low level whereas the all-on control signal AONB, a clocksignal CK1 and a clock signal CK2 turn into the high level, the voltagepotential at the node N2 turns into VDD whereas a voltage potential at anode N1 and the voltage potential at the output terminal OUT turn intoVSS in the unit circuit 61, as in the period t shown in FIG. 6. During aperiod to that the all-on control signal AON, the start pulse ST, theclock signal CK1 and the clock signal CK2 turn into the high levelwhereas the all-on control signal AONB turns into the low level, thevoltage potential at the node N2 turns into VSS, and the voltagepotential at the node N1 and the voltage potential at the outputterminal OUT turn into VDD.

The shift register 60 according to this embodiment can turn all outputsignals SROUT1 to SROUTn into the high level when the all-on controlsignals AON and AONB are supplied externally upon turn-on, testing andthe like.

Seventh Embodiment

aaa. FIG. 19 is a block diagram showing a configuration of a shiftregister according to a seventh embodiment of the present invention. Theshift register 70 shown in FIG. 19 includes n unit circuits 71 eachhaving an initialization terminal INIT, an all-on control terminal AON,an output terminal OUT1 and an output terminal OUT2. The shift register70 corresponds to the shift register 10 according to the firstembodiment to which an output separating function and an all-on functionare added.

In the shift register 70, as in the shift register 50, output signalsOUT1 from the unit circuits 71 are externally output as output signalsSROUT1 to SROUTn. Output signals OUT2 from the unit circuits 71 exceptthe n-th unit circuit 71 are supplied to input terminals IN of thesubsequent unit circuits 71, and an output signal OUT2 from the n-thunit circuit 71 is externally output as a test output signal TESTOUT. Inthe unit circuit 71, the initialization terminal INIT and the all-oncontrol terminal AON are supplied with an initialization signal INIT andan all-on control signal AON each given externally.

FIG. 20 is a circuit diagram of the unit circuit 71 included in theshift register 70. The unit circuit 71 shown in FIG. 20 corresponds tothe unit circuit 51 according to the fifth embodiment to which atransistor T13 is added and in which a part to be connected with asource terminal of a transistor T3 is changed. The transistor T13 isidentical in connection form and function to that according to the thirdembodiment. The source terminal of the transistor T3 is connected to theall-on control terminal AON.

When the initialization signal INIT is at a low level and the all-oncontrol signal AON is at the low level, the transistor T13 turns into anOff state, the source terminal of the transistor T3 is applied with avoltage potential VSS, and the unit circuit 71 operates as in the unitcircuit 11 according to the first embodiment. During a period that theinitialization signal INIT is at a high level, moreover, the transistorT13 turns into an On state, a voltage potential at a node N2 turns intoVDD, and the transistor T3 and a transistor T16 turn into the On state.Herein, when the all-on control signal AON turns into the high level, avoltage potential at the output terminal OUT2 is maintained at VSS, anda voltage potential at the output terminal OUT1 turns into VDD. The samething holds true for a case where clock signals CK1 and CK2 turn intothe high level in place of the initialization signal INIT.

FIG. 21 is a timing chart of the shift register 70. In FIG. 21, during aperiod t that the initialization signal INIT, a start pulse ST and theall-on control signal AON turn into the low level whereas the clocksignal CK1 and the clock signal CK2 turn into the high level, thevoltage potential at the node N2 turns into VDD whereas a voltagepotential at a node N1, the voltage potential at the output terminalOUT1 and the voltage potential at the output terminal OUT2 turn into VSSin the unit circuit 71, as in the period t shown in FIG. 6. During aperiod tb that the initialization signal INIT and the start pulse STturn into the low level whereas the all-on control signal AON, the clocksignal CK1 and the clock signal CK2 turn into the high level, thevoltage potential at the node N2 and the voltage potential at the outputterminal OUT1 turn into VDD whereas the voltage potential at the node N1and the voltage potential at the output terminal OUT2 turn into VSS. Thesame thing holds true for a period tc that the initialization signalINIT and the all-on control signal AON turn into the high level whereasthe start pulse ST, the clock signal CK1 and the clock signal CK2 turninto the low level.

The shift register 70 according to this embodiment can prevent erroneousoperations by outputting the external output signal and the input signalto the subsequent unit circuit from the unit circuit 71 in a separatemanner, and can turn all the output signals SROUT1 to SROUTn into thehigh level by receiving the initialization signal INIT and the all-oncontrol signal AON externally upon turn-on, testing and the like.

Eighth Embodiment

bbb. FIG. 22 is a block diagram showing a configuration of a shiftregister according to an eighth embodiment of the present invention. Theshift register 80 shown in FIG. 22 includes (n+1) unit circuits 81 eachhaving an input terminal INs and an input terminal INr. By use of anoutput signal from a subsequent circuit, the shift register 80 performsdischarge of a node N1 and pull-down of an output signal.

In the shift register 80, a start pulse ST is supplied to the inputterminal INs of the first unit circuit 81. Output signals OUT from theunit circuits 81 except the (n+1)-th unit circuit 81 are supplied to theinput terminals INs of the subsequent unit circuits 81 and the inputterminals INr of the preceding unit circuits 81. An output signal OUT(an output signal “dummy”) from the (n+1)-th unit circuit 81 is suppliedto the input terminal INr of the (n+1)-th unit circuit 81 via a buffer82.

FIG. 23 is a circuit diagram of the unit circuit 81 included in theshift register 80. As shown in FIG. 23, the unit circuit 81 correspondsto the unit circuit 11 according to the first embodiment to whichtransistors T19 and T23 are added and in which parts to be connectedwith gate terminals of transistors T1 and T7 are changed. The transistorT19 has a drain terminal applied with a power supply voltage VDD, and asource terminal connected to a node N2. Moreover, the transistor T23 hasa drain terminal connected to a node N1, and a source terminal connectedto a ground. The gate terminals of the transistors T1 and T7 areconnected to the input terminal INs, and gate terminals of thetransistors T19 and T23 are connected to the input terminal INr. Inorder to stabilize an operation of the unit circuit 81, a transistor T20having a gate terminal connected to the node N1 may be provided betweenthe node N2 and the ground.

During a period that the output signal from the preceding unit circuit81 (the input signal INs) is at a high level, the transistors T1 and T7turn into an On state, the node N1 is applied with a High voltage, andthe node N2 is applied with a Low voltage. During a period that theoutput signal from the subsequent unit circuit (the input signal INr) isat the high level, the transistors T19 and T23 turn into the On state,the node N1 is applied with the Low voltage, and the node N2 is appliedwith the High voltage.

In the unit circuit 81, a reset signal generation circuit configuredwith a transistor T5, a transistor T6, the transistor T7, a capacitor C2and a capacitor C3 generates a reset signal which turns into the highlevel in a normal state, by use of two-phase clock signals CK1 and CK2,and changes the reset signal to a low level when an input signal INturns into the high level. This reset signal is supplied to gateterminals of the transistors T3 and T4, and is used for discharge of thenode N1 and pull-down of the output signal OUT.

The input signal INr is also used for discharge of the node N1 andpull-down of the output signal OUT. When the input signal INr turns intothe high level, the transistor T23 turns into the On state, and avoltage potential at the node N1 turns into VSS. Moreover, when theinput signal INr turns into the high level, the transistor T19 turnsinto the On state, and a voltage potential at the node N2 turns intoVDD. Therefore, the transistor T3 turns into the On state, and a voltagepotential at the output terminal OUT turns into VSS. A circuitconfigured with the transistors T5 and T6 and the capacitors C2 and C3has a function of not allowing the voltage potential at the node N2 torise to VDD, but maintaining the voltage potential at the node N2 atVDD.

FIG. 24 is a timing chart of the shift register 80. The timing chartshown in FIG. 24 is different from the timing chart shown in FIG. 3 inthe following points. In FIG. 3, the voltage potential at the node N2 inthe unit circuit 11 gradually rises from VSS to reach VDD. In FIG. 24,on the other hand, the voltage potential at the node N2 in the unitcircuit 81 changes to VDD immediately when the output signal OUT changesto the low level.

Like the shift register 80 according to this embodiment, it is possibleto perform the discharge of the node N1 and the pull-down of the outputsignal OUT by use of the output signal from the subsequent circuit, togenerate the reset signal at the high level by use of the two-phaseclock signals CK1 and CK2 whose high level periods do not overlap witheach other, and to fix the output signal OUT at the low level by use ofthe reset signal.

With regard to the shift registers according to the first to eighthembodiments, various modification examples may be employed. Hereinafter,as one example, description will be given of a modification example ofthe shift register 10 according to the first embodiment. For example,the unit circuit 11 may include a precharge circuit 15 shown in FIG. 25or a precharge circuit 16 shown in FIG. 26, in place of the prechargecircuit 13 shown in FIG. 2. The gate terminal of the transistor T1 isconnected to a clock terminal CKB in the precharge circuit 15 or isconnected to an input terminal IN in the precharge circuit 16.

Each of the precharge circuits 13, 15 and 16 applies a High voltage tothe node N1 (the gate terminal of the transistor T2) during the periodthat the input signal IN is at the high level. The precharge circuit 13has an effect that a load is light because an input signal is suppliedto a gate terminal of a transistor. The precharge circuit 15 has aneffect that it also functions as a discharge circuit that discharges theelectrical charge stored in the node N1 when the clock signal CKB is atthe high level and the input signal IN is at the low level. Theprecharge circuit 16 has an effect that it can reduce a stress due to agate-drain voltage.

Like a unit circuit 17 shown in FIG. 27, moreover, all of or part ofcapacitors C1 to C3 may be configured using a wiring capacitor or aparasitic capacitor of a transistor, rather than a capacitive element.Thus, it is possible to reduce an amount of circuits because acapacitive element is not provided.

In the unit circuit 11, the voltage potential at the node N1 becomeshigher than VDD; therefore, a high voltage is applied between the drainand the gate of the transistor T1, between the drain and the gate of thetransistor T4, and between the drain and the source of the transistorT4. If the high voltage exceeds a withstand voltage, the transistors T1and T4 are broken down. Even when the high voltage does not exceed thewithstand voltage, it is not preferred for the transistors T1 and T4 tobe continuously applied with the high-voltage stress. Moreover, the highvoltage is applied between the drain and the source, so that an off leakcurrent increases in each of the transistors T1 and T4. As a result,there is a possibility that the voltage potential at the node N1 in thefloating state drops, leading to erroneous operations of the shiftregister 10.

As a countermeasure against the withstand voltage and the off leakcurrent, like a unit circuit 18 shown in FIG. 28, transistors T21 andT22 each having a gate terminal applied with the power supply voltageVDD may be provided between the node N1 and the source terminal of thetransistor T1 and between the node N1 and the drain terminal of thetransistor T4. The unit circuit 18 may include both of or one of thetransistors T21 and T22. In a case where a connection point between thetransistors T1 and T21 is defined as a node N4 and a connection pointbetween the transistors T4 and T22 is defined as a node N5, a voltagepotential at the node N4 and a voltage potential at the node N5constantly become (VDD−Vth) or less (see FIG. 29).

As countermeasures against the off leak current, additionally, there areconsidered a method of extending a gate length of the transistors T1 andT4, a method of configuring each of the transistors T1 and T4 with aplurality of transistors connected in series, a method of configuringeach of the transistors T1 and T4 by use of an LDD (Lightly Doped Drain)structure, and the like.

Like a shift register 90 shown in FIG. 30 corresponding to the shiftregister 10 to which a unit circuit 11 is added as a dummy stage, anoutput signal OUT from the (n+1)-th unit circuit 11 may be output as atest output signal TESTOUT. Thus, it is possible to further reduce aninfluence of noise to be exerted on the test output signal TESTOUT froman external device and to prevent erroneous operations of a shiftregister, as compared with the shift register 50 having the outputseparating function (FIG. 14).

As shown in FIG. 31, moreover, a unit circuit 19 may be configured witha P-channel type transistor, and a shift register having n stages may beconfigured in such a manner that a plurality of unit circuits 19 arecascaded. FIG. 32 is a timing chart of the shift register including theunit circuit 19. The unit circuit 19 is similar in specificconfiguration and operation to the unit circuit 11; therefore,description thereof will not be given here.

A modification example similar to that of the first embodiment may beemployed with regard to the shift registers according to the second toeighth embodiments. A novel shift register may be configured by anoptional combination of the characteristics of the shift registersaccording to the first to eighth embodiments and the modificationexamples thereof as long as such a combination is not against propertiesof the characteristics. Thus, it is possible to obtain a power-savingshift register capable of fixing an output signal at a predeterminedlevel in a normal state without allowing a through current to flowtherein.

INDUSTRIAL APPLICABILITY

The shift register of the present invention can fix an output signal ata predetermined level in a normal state without allowing a throughcurrent to flow therein, and brings about an effect of achieving lowpower consumption. Therefore, the shift register of the presentinvention is applicable as a drive circuit in a display device and animaging device, or the like, for example.

1. A method of operating a shift register which generates a reset signal and an output signal, the reset signal having at least an associated high state and an associated low state, the output signal having at least an associated high state and an associated low state, the method comprising: receiving, by the shift register, an input signal, the input signal having at least an associated high state and an associated low state; changing the associated high state of the reset signal to the associated low state of the reset signal when the associated low state of the input signal changes to the associated high state of the input signal; and changing the associated low state of the reset signal to the associated high state of the reset signal subsequent to the associated high state of the output signal changing to the associated low state of the output signal.
 2. The method of claim 1 further comprising: receiving, by the shift register, a first clock signal and a second clock signal, the first clock signal having at least an associated high state and an associated low state, the second clock signal having at least an associated high state and an associated low state; wherein a period of the associated high state of the input signal partially overlaps a period of the associated low state of the first clock signal and a period of the associated high state of the second clock signal.
 3. The method of claim 2, wherein the shift register includes a unit circuit having a reset signal generation circuit including a first capacitor, further comprising: charging and discharging the first capacitor based on the first clock signal.
 4. The method of claim 3, wherein the reset signal generation circuit further includes a second capacitor, further comprising: changing the associated high state of the reset signal to the associated low state of the reset signal based on an electrical charge stored in the second capacitor.
 5. The method of claim 1, further comprising: first changing a voltage at a first node of the unit circuit from a first voltage to a second voltage upon receipt of the input signal, second changing the voltage at the first node from the second voltage to the third voltage after the first changing, the third voltage being higher than the second voltage, and third changing the voltage at the first node from the third voltage to the second voltage after the second changing, when the reset signal is at the associated low state.
 6. The method of claim 5, further comprising: receiving, by the shift register, a first clock signal and a second clock signal, the first clock signal having at least an associated high state and an associated low state, the second clock signal having at least an associated high state and an associated low state; wherein the third changing occurs when the second clock signal is at the associated low state, and the associated high state of the first clock signal changes to the associated low state of the first clock signal.
 7. The method of claim 5, further comprising: changing the associated high state of the output signal to the associated low state of the output signal upon the third changing.
 8. The method of claim 1, further comprising: first changing a voltage at a first node of the unit circuit from a first voltage to a second voltage upon receipt of the input signal, second changing the voltage at the first node from the second voltage to the third voltage after the first changing, the third voltage being higher than the second voltage, and third changing the voltage at the first node from the third voltage to the second voltage after the second changing, and changing the associated high state of the output signal to the associated low state of the output signal upon the third changing.
 9. The method of claim 8, further comprising: receiving, by the shift register, a first clock signal and a second clock signal, the first clock signal having at least an associated high state and an associated low state, the second clock signal having at least an associated high state and an associated low state; wherein the third changing occurs when the second clock signal is at the associated low state, and the associated high state of the first clock signal changes to the associated low state of the first clock signal.
 10. The method of claim 1, further comprising: receiving, by the shift register, a first clock signal and a second clock signal, the first clock signal having at least an associated high state and an associated low state, the second clock signal having at least an associated high state and an associated low state, and changing the associated low state of the reset signal to the associated high state of the reset signal when the first clock signal is at the associated low state, and the associated low state of the second clock signal changes to the associated high state of the second clock signal.
 11. A shift register comprising: a plurality of unit circuits configured to operate based on an input signal, the input signal having at least an associated high state and an associated low state, at least one of the plurality of unit circuits including, a reset signal generation circuit configured to generate a reset signal such that an associated high state of the reset signal changes to an associated low state of the reset signal if the associated low state of the input signal changes to the associated high state of the input signal, and first and second transistors coupled at an output node, the output node configured to output an output signal, the output signal having at least an associated high state and an associated low state, wherein the reset generation circuit is configured to change the associated low state of the reset signal to the associated high state of the reset signal subsequent to the associated high state of the output signal changing to the associated low state of the output signal.
 12. The shift register of claim 11, wherein the reset signal generation circuit is configured to, receive a first clock signal and a second clock signal, the first clock signal having at least an associated high state and an associated low state, the second clock signal having at least an associated high state and an associated low state, change the associated low state of the reset signal to the associated high state of the reset signal when the first clock signal is at the associated low state, and the associated low state of the second clock signal changes to the associated high state of the second clock signal.
 13. The shift register of claim 12, wherein the first transistor is configured to receive the first clock signal and the second transistor is connected to ground.
 14. The shift register of claim 12, wherein the reset signal generation circuit includes a first capacitor and the reset signal generation circuit is configured to, discharge the first capacitor based on at least one of the first and second clock signals.
 15. The shift register of claim 14, wherein the reset signal generation circuit includes a second capacitor the reset signal generation circuit is configured to, change the reset signal based on an electrical charge stored in the second capacitor.
 16. The shift register of claim 11, wherein the first transistor is configured to receive a first clock signal and the second transistor is connected to ground.
 17. The shift register of claim 11, wherein the reset signal generation circuit is configured to receive a first clock signal and a second clock signal, the first clock signal having at least an associated high state and an associated low state, the second clock signal having at least an associated high state and an associated low state, the associated high state of the output signal changing to the associated low state of the output signal when the second clock signal is at the associated low state, and the associated high state of the first clock signal changes to the associated low state of the first clock signal.
 18. A shift register comprising: a plurality of unit circuits configured to operate based on two-phase clock signals, each of the unit circuits including, an output control transistor including a first conduction terminal configured to receive a first clock signal of the two-phase clock signals, and a second conduction terminal connected to an output terminal, a precharge circuit coupled to a control terminal of the output control transistor, a reset signal generation circuit configured to generate a reset signal based on the two-phase clock signals, the reset signal generation circuit further configured to provide the reset signal to a node, and a discharge circuit configured to deactivate the output control transistor, wherein the reset signal generation circuit is configured to change a voltage level of the node upon receipt of an input signal and subsequent to the output signal changing states.
 19. The shift register of claim 18, wherein the reset signal generation circuit includes, a charge storage element, a first transistor configured to receive the first clock signal and charge the charge storage element based on the first clock signal, and a second transistor configured to receive a second clock signal of the two-phase clock signals and change the voltage level of the node if the charge storage element is charged.
 20. The shift register of claim 18, wherein the reset signal generation circuit is configured to receive the first clock signal and a second clock signal of the two-phase clock signals, the first clock signal having at least an associated high state and an associated low state, the second clock signal having at least an associated high state and an associated low state, the associated high state of the output signal changing to the associated low state of the output signal when the second clock signal is at the associated low state, and the associated high state of the first clock signal changes to the associated low state of the first clock signal. 